A deadly plague bacterium is able to transform the lungs into a breeding ground for other microbes—often escaping detection until it is too late for medical treatment.

Most other microbes that infect the lungs trigger an antimicrobial response within a few hours after infection. This early inflammatory response is generally sufficient to eliminate microorganisms with no more than mild respiratory symptoms. But the pneumonic plague bacterium, Yersinia pestis, hides out for about 36 hours when the lungs are “quiet,” not inflamed, and symptoms are completely absent.

During that first 36 hours of infection, plague bacteria are having a field day, growing and reproducing rapidly—2-fold, 100-fold, 100,000-fold—and all without outward disease symptoms or measurable changes in lung tissue.